Historically, ATM (Asynchronous Transfer Mode) has been a cell-based transport protocol, without any significant knowledge or understanding of the underlying frame (also referred to as packet) structure that is being transmitted. Technology advances have taken a path whereby more and more functions are being moved to frame-based services. The fixed length cells that have made ATM such a deterministic transport are now becoming a liability with frame-based services and transports.
ATM has always had more overhead than most other protocols. Although the ATM cell header is only 5 bytes, this overhead is imposed on each cell. Given that cells are only 53 bytes long, this amounts to almost 10% overhead. This fixed overhead is often referred to as a “cell tax”. For large frames, 1000 bytes and more, this can amount to over a hundred bytes of cell headers. In addition, cells must be padded out to a total of 48 payload bytes. While the overhead for large frames can be about 10%, the real problems happen with small frames. Aside from the cell tax, for small frames the required cell padding can account for as much as 40% additional overhead. A 60 byte frame therefore requires 2 cells worth of payload, with the second cell including 12 bytes of real data and 36 bytes of padding.
Up until recently, normal traffic mixes would only generate a small percentage of these tiny data frames. However, the advent of Voice over IP (VoIP) is creating a substantial increase in packetized voice traffic and its resultant small frames. This becomes a problem when a cell-based transport is combined with a frame-based transport.
FR (Frame Relay) to ATM (FR/ATM) intercommunications is accomplished by linking the virtual connections of a frame relay interface with that of an ATM interface. Frame relay is a frame-based transport. Part of the function performed by the FR and ATM switches is the conversion of ATM cells to frames and frames to ATM cells. This function is dependent on the definition of the virtual connection. Because ATM transport has more overhead than frame relay, the typical guidelines for creating an FR/ATM connection is to provide more bandwidth on the ATM side of the connection to account for this added overhead.
Historically, this asymmetry in FR/ATM provisioning has been based on the generally acceptable mixtures of small, medium, and large data frames as found on the Internet. Known as the IMIX (Internet MIXture), the resultant provisioning guidelines typically call for 12 to 18% extra bandwidth on the ATM side of the interface. These guidelines, while appropriate for IMIX traffic, does not work well for traffic flows that are mixed voice and data, or predominantly VoIP. VoIP traffic can require anywhere from 15 to 110% extra bandwidth on the ATM side of a FR/ATM connection. Although this can be a well defined quantity, depending upon the VoIP CODEC (Coder/Decoder) and sampling interval, any change in these parameters can greatly impact the asymmetry percentage. Mixed voice and data traffic presents other problems that are also found with varying CODECs and sampling intervals.
The problem with mixed voice and data traffic is that on the frame relay side of the connection if there is insufficient ATM bandwidth it will not be possible for the ATM network to generate the necessary number of cells to carry the frames supplied by the frame relay network. This is especially true for small frames that will generate a proportionally larger number of cells greater than the ATM network can support. The converse is also true of large frames that are presented to the ATM network. If the asymmetry percentage is too high and the ATM bandwidth is much larger than the frame relay bandwidth, large frames will be packaged in cells with little overhead or padding. As these cells are converted back to frames, there will be more frames generated than can be handled by the frame relay network. Either of these scenarios can result in data losses that can impact customer services. While it is possible to optimize the FR/ATM interface for either large or small frames, it is currently not possible to do both at the same time.